go-fdpass-demo

Demo go code showing how to pass a file descriptor between processes using Unix Domain Sockets.

Since fd-passing is a fairly arcane part of Unix and perhaps an even more arcane part of go, I figure that others might benefit from having access to a known working example. While there are plenty of available examples written in C, at the time of writing I could not readily find a working example in go. Thus this is that.

When might you use fd-passing?

There are a number of use-cases, but generally fd-passing is used when you want to centrally manage and control access to underlying system resources in a way that is not readily possible with the controls offered by the operating system.

For example you might want to give clients access to sockets created on privileged ports but only to some clients and only to some ports. One way to do this is have a server establish the socket and fd-pass it back to the client if it passes the access-control rules.

Another example might be if you want to give clients access to some files in a directory but not others. Such as those under a certain size or age. The client sends the open request to the server, the server applies the age/size logic and fd-passes back the opened file if it's is approved.

Another use-case is to create a server as a container of idle network connections. If your main server uses a lot of state per connection and cannot easily be modified then a small modification to the main server might be to fd-pass idle sockets to the container server which monitors for activity and then fd-passes active sockets back to the heavy-state server.

To be fair, the number of use-cases are not large and some use-cases might be implemented just as easily with fuse or similar. But when you do have a use-case, now you have a guide to get you up and running.

How to use

1. Run make

2. Run ./server in one terminal

3. Run ./client in another terminal

4. Type some lines of text into the ./client terminal

5. Text should show up on the ./server terminal

Runs on?

This demo is know to work on Linux, FreeBSD and macOS using go1.11.6 and beyond.

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Search terms: go, golang, fd-passing, CMSG, Control Message, SCM_RIGHTS, recvmsg, sendmsg, Unix.

Copyright and License

go-fdpass-demo is Copyright ©️ 2020 Mark Delany. This software is licensed under the BSD 2-Clause "Simplified" License.